In an electrical discharge machining apparatus that machines a workpiece, while spraying the machining fluid thereonto or immersing the workpiece in the machining fluid, and applying pulse voltages between the workpiece and an electrode so as to generate electrical discharge, when water is utilized as the machining fluid, electrical insulation properties are required therefor, so that ion exchanged water is usually used. And then, as for this ion exchanged water (machining fluid), tap water is passed through an ion exchange resin that is formed of a mixture of, for example, a cation exchange resin of H+ (hydrogen ion) type and an anion exchange resin of OH− (hydroxide ion) type, so that machining fluid required for electrical discharge machining is produced from which contaminants have been removed.
Note that, in the ion exchange resin, cations such as sodium ions (Na+) and/or calcium ions (Ca2+) contained in the tap water are exchanged for H+ by coming in contact with the cation exchange resin of H+ type, and anions such as chloride ions (Cl−) and/or sulfate ions (SO42−) are exchanged for OH− by coming in contact with the anion exchange resin of OH− type; as a result, contaminants contained in the tap water are removed, and H+ and OH− are coupled, so that water H2O is produced.
In an electrical discharge machining apparatus using such an aqueous machining fluid, it is known that, when a workpiece is immersed in the aqueous machining fluid for a long time, the workpiece is corroded, resulting in lowering product quality of the workpiece.
For this reason, a method of preventing corrosion of a workpiece is proposed in which anti-corrosion electrodes are mounted to the workpiece and to a location adjacent to the workpiece, voltage is applied across the workpiece and the anti-corrosion electrodes, and then a mean voltage at the workpiece side is controlled so as to be zero or negative. (For example, refer to Patent Document 1.)
In addition, a method of preventing corrosion of a workpiece is also proposed in which, by intervening an insulating material between a machine platen and the workpiece, voltage is applied across the machine platen as an anode and the workpiece as a cathode. (For example, refer to Patent Document 2.)    [Patent Document 1] Japanese Patent Application Publication No. H11-70414, and    [Patent Document 2] Japanese Patent Application Publication No. 2004-291206.